Time division duplex and time division multiple access protocols are frequently used in digital mobile and cordless communication systems in order to provide duplex channels on a single radio frequency carrier and in order to use a single rf carrier to provide many voice channels. One problem with such protocols is that as the radio frequency signal is switched on or off, the frequency spectrum of the signal momentarily widens which causes interference on adjacent radio frequency carriers. The momentary widening of the frequency spectrum is frequently known as "splatter".
In order to overcome or reduce this problem, the amplitude profile of the radio frequency carrier must be carefully shaped as the transmitter is switched on. This shaping is normally done in the power amplifier by coupling out a portion of the output signal, which after detection is compared with a reference signal and the difference is low pass filtered to form an "error" signal which is used to control the gain of the power amplifier. The reference signal which provides the shaping may be derived from a series of digital values which are read out from a ROM and which are applied to a digital to analogue converter. Such an arrangement is disclosed in FIG. 5 of UK Patent Specification GB-A-2 220 808. Whilst such known arrangements function satisfactorily they are relatively costly and consume power which is a disadvantage with battery powered equipment.